Quarterly taxes without the penalty
Freelance income, a vesting cliff, an S-corp draw, a Roth conversion — when
income is lumpy, withholding alone rarely covers it. /v1/tax/estimated-tax
finds the §6654 safe harbor that governs, the quarterly schedule that satisfies it, and the
underpayment penalty on the current trajectory.
1 · Make the call
A single filer whose income jumped: $20,000 of tax last year (AGI $200,000 — so the 110% prior-year harbor applies), $30,000 projected this year, $8,000 of withholding, and $2,000 paid in each of the first two quarters.
curl -X POST https://api.numeratica.com/v1/tax/estimated-tax \
-H "Authorization: Bearer nmr_sk_test_xxx" \
-H "Content-Type: application/json" \
-d '{
"tax_year": 2026,
"filing_status": "single",
"prior_year_tax": 20000,
"prior_year_agi": 200000,
"projected_current_tax": 30000,
"withholding": 8000,
"estimated_payments": [2000, 2000, 0, 0]
}'2 · The response
{
"result_id": "esttax_437654e24a0c1d7e",
"engine_version": "estimated-tax-1.0.0",
"result": {
"current_year_safe_harbor": 27000, /* 90% of projected tax */
"prior_year_safe_harbor": 22000, /* 110% of last year (AGI > $150k) */
"governing_safe_harbor": "prior_year", /* pay the lesser */
"required_annual_payment": 22000,
"total_paid": 12000,
"shortfall": 10000,
"safe_harbor_met": false,
"required_quarterly": 5500,
"additional_per_quarter_needed": 3500,
"installments": [
{ "quarter": 1, "due_date": "04-15", "required": 5500, "paid": 4000,
"underpaid": 1500, "penalty": 17.55 },
{ "quarter": 2, "due_date": "06-15", "required": 5500, "paid": 4000,
"underpaid": 3000, "penalty": 52.93 }, /* running underpayment — §6654(b)(3) */
{ "quarter": 3, "due_date": "09-15", "required": 5500, "paid": 2000,
"underpaid": 6500, "penalty": 152.08 },
{ "quarter": 4, "due_date": "01-15", "required": 5500, "paid": 2000,
"underpaid": 10000, "penalty": 172.60 }
],
"penalty_rate": 0.07,
"estimated_penalty": 395.16
},
"disclaimer": "Calculation engine output for informational purposes only; not tax or financial advice."
}The cheaper harbor is 110% of last year: $22,000 — not 90% of this year's $30,000. On the current trajectory the year ends $10,000 short with a $395 penalty; catching up means $3,500 more per remaining quarter. Withholding counts as paid evenly through the year, which is why a Q4 withholding bump (a bonus, an extra W-2 election) beats a late estimated payment.
3 · Render it
The quarter-by-quarter card a dashboard needs.
<div style="font-family:-apple-system,Segoe UI,Roboto,Helvetica,Arial,sans-serif;color:#1a2230;max-width:600px">
<div style="font-size:14px;color:#5b6675;margin-bottom:8px">§6654 safe harbor: 110% of prior year · $22,000 required · $5,500/quarter</div>
<div style="display:grid;grid-template-columns:70px 1fr 90px 90px;gap:6px;font-size:13px;align-items:center">
<div style="color:#5b6675">Q1 04-15</div><div style="background:#e8edf8;border-radius:4px"><div style="width:73%;background:#b45309;height:16px;border-radius:4px"></div></div><div style="text-align:right">$4,000</div><div style="text-align:right;color:#dc2626">−$1,500</div>
<div style="color:#5b6675">Q2 06-15</div><div style="background:#e8edf8;border-radius:4px"><div style="width:73%;background:#b45309;height:16px;border-radius:4px"></div></div><div style="text-align:right">$4,000</div><div style="text-align:right;color:#dc2626">−$1,500</div>
<div style="color:#5b6675">Q3 09-15</div><div style="background:#e8edf8;border-radius:4px"><div style="width:36%;background:#dc2626;height:16px;border-radius:4px"></div></div><div style="text-align:right">$2,000</div><div style="text-align:right;color:#dc2626">−$3,500</div>
<div style="color:#5b6675">Q4 01-15</div><div style="background:#e8edf8;border-radius:4px"><div style="width:36%;background:#dc2626;height:16px;border-radius:4px"></div></div><div style="text-align:right">$2,000</div><div style="text-align:right;color:#dc2626">−$3,500</div>
</div>
<div style="margin-top:12px;border:2px solid #dc2626;border-radius:10px;padding:12px;display:flex;justify-content:space-between;align-items:baseline">
<div><strong>On this trajectory</strong><span style="color:#5b6675;font-size:13px"> · $10,000 short of the harbor</span></div>
<div style="font-size:22px;font-weight:700;color:#dc2626">$395 penalty</div>
</div>
<div style="margin-top:8px;font-size:14px">Fix: <strong>+$3,500</strong> in each remaining quarter — or a Q4 withholding bump, which counts as paid all year.</div>
<div style="color:#5b6675;font-size:12px;margin-top:6px">result_id esttax_437654e24a0c1d7e · 7% IRS rate (overridable) · not tax advice</div>
</div>
Where the lumpy income comes from:
/v1/tax/s-corp-salary for owner comp,
ISO exercises & AMT, and
Roth conversions — all of which this endpoint keeps
penalty-free. Self-employed? /v1/retirement/self-employed
sizes the solo-401(k)/SEP contribution that shrinks the projected tax in the first place.